You should get the normal Cheevo for killing him, but not the STRIKE TEAM cheivment, which is for pre-Patch 5.9 kill, and to be eligible for the Title and Mount legacy wide.
This seems to be bugged due to the delayed patch.
Sold a kill pre-weekly reset, and that person got all the correct achievements, but saw a few kills today(post reset) and they didnt get it. (only got the normal cheevo, no STRIKE TEAM cheevo)